The human eye in its simplest terms functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of a lens onto a retina. The quality of the focused image depends on many factors, including the size and shape of the eye, and the transparency of the cornea and lens. Age and/or disease often cause the lens to become less transparent. Thus, vision deteriorates because of the diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is medically known as a cataract.
An accepted treatment for this condition is surgical removal of the lens and replacement of the lens function by an IOL. For many years most IOLs were made of poly (methylmethacrylate), a material with good optical characteristics and compatibility with the tissues of the eye. A disadvantage of PMMA is, however, that it is a very rigid material and the incision must be made large enough for implantation of the IOL. If the optical properties are not correctly matched, a need for a second IOL is required.
All incisions in the eye are accompanied by trauma, and so, although foldable lenses have been a great improvement, there is still a need for an IOL that can be adjusted without an additional incision. Complications of conventional IOL implantation, namely decentration and posterior capsular opacification, may also require adjustment.
Hydrogels are a class of materials that are very interesting for an injectable lens because they have the added advantage that their aqueous composition approximates that of the natural lens. Hydrogels can be made by crosslinking aqueous polymer or monomer/crosslinker solutions. Since monomers are often toxic, the use of polymers is preferred for applications in the eye. Polymers, to which a reactive group is attached, for example, an acrylate group, can be polymerized in the presence of water and form a hydrogel. However, the injectable lens may also not have the desired optical properties.